Nonaqueous electrolyte secondary batteries such as a lithium secondary battery are currently in wide use as (i) batteries for devices such as a personal computer, a mobile telephone, and a portable information terminal or (ii) on-vehicle batteries.
As a separator for use in such a nonaqueous electrolyte secondary battery, a porous film containing polyolefin as a main component is mainly used.
For example, Patent Literature 1 discloses, as a battery separator excellent in input/output characteristics and in safety, a battery separator constituted by a porous film whose thickness and resin occupancy, which is calculated based on a cross sectional image obtained with use of a scanning electron microscope (SEM), are specified within specific ranges.